sh: Move board specific options into the Board support menu

Move the sourcing of the board specific Kconfig files into the
"Board support" menu. Without this they appear underneath the
"Board support" menu, in the "System type" menu.

[lethal@linux-sh.org: handle the magicpanelr2 case, too]
Signed-off-by: Stuart Menefy <stuart.menefy@st.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>